ECOLOGICAL STUDY OF BRUSH LAKE. 163, 
GENERAL CONSIDERATIONS. 
That the plants around the lake are arranged in rather defin- 
ite zones appears to be due to two general causes: first, because 
of the gradual change in the character of the substratum and 
other factors from one condition to another; second, because 
certain plants are especially adapted to certain conditions and are 
thus able to hold their ground against invaiders from the outside. 
In an ordinary pond the successive modifications of the sub- 
stratum occur in concentric belts and hence the succession of 
more or less perfect zones or belts of vegetation. Between each 
two contiguous zones there is a tension line where the struggle 
for existence among the opposing individuals becomes very 
severe. Each zone, therefore, has a fighting line on its inner 
and outer edge, while in the central part the struggle is only 
between plants of the community, both old and young, and such 
chance strangers which may be able to gain a foothold in the less 
densely populated areas. That many plants almost entirely 
restricted to a single zone can hold their own, either farther in 
or out, if once established is readily seen where normal conditions 
have been disturbed. ‘There may thus be considerable difference 
in the result, depending on whether plants have had the oppor- 
tunity of occupying a bare soil or whether they must contend for 
the soil already occupied by others. Certain species might hold 
their own indefinitely in a certain environment 1f once firmly 
established, while they might not be able to gain a foothold if 
the soil is already occupied by others. 
The seven zones of Brush Lake are slowly traveling inward, 
as the lake is filling up with the wash from the surrounding hills. 
Each zone follows up its ideal environment and is crowded out 
on the outer margin by its next neighbor. ‘The filling must have 
been quite slow and gradual in former times, but at present it is 
rather cataclysmic and will continue to be so inthe future. There 
are therefore unusual disturbances in the progression of the zonal 
societies which have been alluded to above. The physiographic 
changes determine the changes in the zones. If these are gradual 
and slow in their progression the same will be true of the plant 
societies, but if the filling is sudden and large additions are made 
